Add a linter for undocumented telemetry events and keys

## Summary
Add automated validation that telemetry event names and attribute keys are documented wherever they are defined or emitted.

## Goals
- Detect new or renamed telemetry events and keys that lack the required documentation.
- Define the authoritative source(s) for event/key declarations and documentation.
- Report actionable errors identifying the missing event or key and expected documentation location.
- Integrate the check into the appropriate CI/lint workflow.
- Document how the linter works and how to update telemetry documentation when adding a field.

The check should cover core telemetry fields/events and the applicable extension telemetry documentation without requiring runtime telemetry collection.

## Reference
Follow-up from [PR #9174 review comment](https://github.com/Azure/azure-dev/pull/9174#discussion_r3754754796).